The present invention relates to a method for slitting a magnetic tape, and particularly relates to a method for slitting a magnetic tape in which a wide magnetic tape is cut into narrow magnetic tapes while the wide magnetic tape is continuously run.
Recently, magnetic tapes or the like widely used in various fields have been produced by a method in which a magnetic tape having a width wider than that of the final product is subjected to various treatments and then cut into separate tapes of the final product width.
Such slitting of a magnetic tape is performed continuously while the tape is pulled from a wider magnetic tape master roll and continuously run. A slitter used in this slitting method employs a plurality of rotating circular blades (hereinafter referred to as "rotary blades") having rotational axes arranged substantially parallel to the tape surface of a magnetic tape and extending in the tape width direction, the rotary blades being arranged side by side in upper and lower pairs so that each pair of upper and lower opposing rotary blades radially overlap each other from the front and rear side surfaces of the magnetic tape.
A conventional slitting method will be described with reference to FIGS. 9 to 11. FIG. 9 is a schematic side view, FIG. 10 is a sectional view taken along a line C--C in FIG. 9, and FIG. 11 is a sectional view taken along a line D--D in FIG. 9.
As shown in FIG. 9, upper and lower blades 20 and 30, which are rotary blades of a slitter 40, rotate in the forward direction of the running direction of a magnetic tape T and about the rotating axes O.sub.1 and O.sub.2 substantially parallel to the tape surface and along the tape width direction, and are arranged opposed to each other from the top and bottom surfaces of the tape so as to overlap each other in their radial directions.
The upper and lower blades 20 and 30 overlap each other to such an extent as to press against each other, whereby slitting is performed by making the above-mentioned magnetic tape pass therebetween.
In such a slitting process, it is known that, as shown in FIG. 10, cracks tend to occur in a magnetic layer and a support 6 in the vicinity of the magnetic tape T, which cracks can come together to cause shearing or separation in the magnetic tape T. (See, for example, Tadaaki Sugita et al., "STUDY ON MICRO-SHEARING PROCESSING OF A THIN FILM," p. 371, Extended Abstracts, The Autumn Meeting, 1984, The Japan Society of Precision Engineering).
In such a conventional slitting method, as shown in FIGS. 10 and 11, in the vicinity of the cut-section there is formed a section A.sub.1 in which the boundary between the support 6 and the magnetic layer 7 located above the lower rotary blade 30 is sheared down to the rear surface side of the tape (downward in the drawings) in the vicinity of the cut-section, while in a section B.sub.1 located under the upper rotary blade 20, on the contrary, there occur burrs where a portion 9 of the magnetic layer 7 is formed tapering away.
Detailed inspection of the burr shows that sometimes the top end of the burr is warped toward the magnetic tape surface. The burr is sheared away, for example, while the tape is guided by a tape edge limiting guide roller or the like acting as a tape guide in a tape carrier system, a magnetic tape cassette assembling apparatus, a recording/reproducing apparatus, or the like. There has thus been a problem in that sheared burr material contaminates the running system or peripheral equipment, or in a recording/reproducing apparatus the sheared burr material not only adheres to the tape running system or other parts of the apparatus, but also fouls the recording/reproducing head or causes data drop out.
On the other hand, generally in the slitting of metal products in a conventional "well shearing" processing, it is known that shearing can be carried out by setting the gap (distance in the direction perpendicular to the shearing section) between blade edges of upper and lower blades so as to cause cracks produced from the upper and lower directions to just come together. Generally in the case of a non-metal, it has been considered that well shearing can be performed by making this gap as small as possible. Since the burr height of the shearing section increases as the blade edge wears, the service life of the blade edge has been judged to end when this height is over a certain value.
It has been found, however, that when the above-mentioned rotary blades 20 and 30 are new or when their blade edges are renewed by grinding, the above-mentioned burr becomes more prominent in relation to the sharpness of the blades. That is, the applicants have found that the sharper the rotary blades 20 and 30, the more conspicuous the above-mentioned problem becomes.
In consideration of the above problems, methods of preventing the above-mentioned burrs from being formed have been disclosed, for instance, in Japanese Unexamined Patent Publications Nos. Sho. 62-86530 and Hei. 1-246094.
The magnetic recording medium disclosed in the former Publication is slit so that its magnetic layer is sheared down to the support side. However, for example, in the case of forming the edge of the thus-arranged tape as a slit section, rubbing of cutting devices (blade edges) such as rotary blades against each other becomes important because a comparatively large load is required on the cutting device. Thus, not only does such rubbing make the life of the cutting device short, but also the setting of the quantity of shearing is difficult.
In the latter Publication, there are proposed rotary blades for slitting a web of a photographic film, a magnetic tape, etc. These rotary blades have a feature that at least one of the blade edges of the upper and lower blades is chamfered so as to gradually part from the other blade radially outward.
It is said that the web slit by rotary blades having such a configuration as described above is rough in its slit section in the web thickness direction. However, the present applicants have found through detailed studies that although the above-mentioned roughness occurs to some extent, sometimes few such effects are produced, depending on the amount of chamfering. They also have found that the problem of burring in the magnetic layer cannot be solved only by forming the above-mentioned roughness.
A shearing theory, much like that applicable to metal shearing processes, can be applied to a web such as a magnetic tape. However, for practical mass production, it has been impossible to apply metal shearing theory without modification since a magnetic tape differs significantly from metal in material and other respects.
As the result of intensive studies, it has been found that the above-mentioned burr formation is produced by a mechanism as will now be described.
If the gap between the upper and lower blade side surfaces 28 and 39 of the upper and lower rotary blades 20 and 30 where they overlap is narrower than a certain range, cracking of the magnetic layer 7, which is the start point of shearing, is produced at the initial stage of shearing at a position closer to the lower blade side surface 39 (the left side in the drawing) from a line extending from the lower blade side surface 39. Therefore the magnetic layer section 9 on the slit section B.sub.1 is made to project to the side (to the section A.sub.1 side). Then, at the point where the upper and lower rotary blades 20 and 30 overlap each other radially in the shearing process, the magnetic layer section 9 is pressed against the lower blade side surface 39 and displaced on the lower blade side surface 39 while rubbing thereagainst (pressed downward in the drawing). As a result, the magnetic layer section 9 is bent up toward the reverse side opposite to the support 6, thus forming a burr.
Thus, by applying a shearing theory which is essentially the same as that for metal shearing processing in the above-mentioned manner, it has been impossible to surely suppress the generation of burrs.